#include <bits/stdc++.h>

int ri() {
	int n;
	scanf("%d", &n);
	return n;
}

int main() {
	int n = ri();
	int a[n];
	for (auto &i : a) i = ri() - 1;
	
	std::vector<int> compressed;
	for (int i = 0; i < n; i++) if (!i || a[i - 1] != a[i]) compressed.push_back(a[i]);
	
	bool possible = true;
	int cnt[n];
	memset(cnt, 0, sizeof(cnt));
	std::vector<int> twos;
	for (int i = 0; i < (int) compressed.size(); i++) {
		if (++cnt[compressed[i]] >= 3) possible = false;
		if (cnt[compressed[i]] == 2) twos.push_back(i);
	}
	for (auto i : twos) if (i && i + 1 < (int) compressed.size()) possible = false;
	
	puts(possible ? twos.size() ? "1" : "0" : "-1");
	
	return 0;
}